The invention relates generally to medical implants and more particularly to medical implants including surgical tubing or cannulae, for catheters, stents, and other devices.
In certain medical procedures, medical implants are placed into the body. These implants include catheters inserted into body passages, vessels, or cavities for passing fluids, draining fluids, making examinations, etc. A stent is a second type of medical implant used to maintain a bodily orifice or cavity during skin grafting or to provide support for tubular structures, for example, during or after anastomosis.
It is generally desirable that medical implants, such as catheters and stents, be radiographically opaque,such that their precise location within the host body can be detected by X-ray examination. In addition, it is advantageous that such medical implant be optically transparent so that a flow of fluid therethrough may be observed.
Many tubular-shaped medical implants, such as catheters and stents are made from a polymer base. The polymers are chosen that can be formed into tubular shapes that are, particularly in the case of catheters, flexible enough to be routed or snaked, to a location in the body. In the case of a peripherally inserted central catheter (PICC), for example, the tubing of the catheter is routed or snaked, in one instance, through a vein in a patient""s arm or neck to the superior vena cava of the patient""s heart. The tubing must be flexible enough to be routed in this manner without causing trauma to the patient. The polymer chosen as the medical implant should also have sufficient strength when formed into a tubing so that the lumen does not collapse in a passageway or orifice. Still further, the tubing should be resistant to crimping or kinking so that a continuous passageway is assured. Thermoplastic polyurethane-based polymers are a popular choice for medical implant polymers.
In general, polyurethanes are condensation products of reactions between diisocyanate (isocyanate compounds having a functionality of two) and soft-block polyols. Typically, polyurethanes are combined with low molecular weight aliphatic or aromatic diols or diamines as chain extenders to impart the useful elastomeric properties of flexibility, strength, and kink resistance. Low molecular weight diols include butane diol, pentane diol, hexane diol, heptane diol, benzene dimethanol, hydraquinone diethanol and ethylene glycol. Diamines include ethylene diamine, butanediamine, propane diamine and pentane diamine. The diamine based compounds generally form a class of polyurethanes called polyurethaneureas. An added feature of these polyurethanes made with diol or diamine chain extenders is that catheters, stents or vascular grafts formed from these materials are typically optically transparent making these polymer matrices excellent compounds for medical implants. Unfortunately, however, these polyurethanes are generally not radiopaque.
Radiopaque medical implants such as catheters, including radiopaque polyurethanes, have been developed. These radiopaque polymer structures are generally of two forms. A first form of radiopaque polymer incorporates a radiopaque filler or pigment. Typical filler materials include barium sulfate (BaSO4), bismuth subcarbonate, or certain metals such as tungsten (W). Other radiopaque fillers are pigments for incorporation into a polymer tube include bismuth oxychloride and other bismuth salts such as bismuth subnitrate and bismuthoxide (See U.S. Pat. No. 3,618,614). A drawback of the filler incorporated polymers is, although such polymers are radiopaque, the filler tends to make the polymer non-transparent.
A second form of radiopaque polymer useful in medical implants incorporates a halogenated-chain extender into the polymer matrix. Examples of these types of polymers are described in U.S. Pat. Nos. 4,722,344; 5,177,170; and 5,346,981. The preferred halogen in these patents is bromine (Br). Polymers incorporating a brominated-chain extender into the polymer matrix generally yield a tubing that is radiopaque and optically transparent.
In order to impart useful radiopaque properties, the halogenated-chain extended polymer, such as a bromine-chain extended polymer, must have a minimum amount of halogen (i.e., bromine) to impart radiopacity to the polymer. Experimental studies show that the minimum amount of bromine, for example, in a polyurethane-based polymer useful as a catheter, is approximately 15 percent. Amounts less than this tend to make the tubing difficult to detect by X-ray.
A second problem with halogenated-chain extended polymers is the maximum amount of halogen that can be incorporated into the polymer is limited. Experimental studies have shown that polymers having, for example, a bromine concentration greater than 30 percent are too stiff for use as a medical implant, such as a catheter tubing. Accordingly, the radiopacity of the tubing is limited by the amount of bromine that may be incorporated in the polymer matrix without degrading the properties of the tubing made from such a polymer.
As noted above, certain halogenated-chain extended polymers offer both radiopacity and optical transparency. However, in order to maintain the superior elasatomeric properties demonstrated by conventional thermoplastic polyurethane with non-halogenated-chain extenders, the mounts of halogen must be strictly limited. It would be desirable, in certain instances, to have a halogenated-chain extended polymer with a radiopaque property that is not limited by the amount of bromine that is incorporated into the polymer matrix. What is needed is a combination that can maximize the radiopacity of the implant without increasing the halogen concentration of the polymer beyond that which would negatively affect the physical characteristics of the medical implant.
A method and apparatus to provide improved radiopacity while retaining optical transparency is described. In one embodiment, a tubing is described having a first polyurethane layer and a second brominated polyurethane layer coupled to the first layer.